This invention relates to a multi-port microwave coupler particularly, but not exclusively, to be used as a part of a beam-forming network for a multi-beam antenna carried by a satellite.
Such multi-port microwave couplers are well-known in the art of microwave frequency transmission and typically comprise a hybrid coupler having four ports, that is two input ports and two output ports. Such hybrid couplers are commonly referred to as 2.times.2 hybrid couplers and have the following characteristics:
1. When a microwave signal is applied to one of the input ports, the complex voltages appearing at both output ports are equal in amplitude, and no power appears at the other input port.
2. When equal-amplitude microwave signals are applied to both of the input ports, all of the power can be made to appear at only one of the output ports by appropriately selecting the relative phases of the two input signals.
However there is a requirement for higher-order couplers in certain applications, for example in beam-forming networks and multiple matrix amplifiers for multi-beam antennas. Such higher-order couplers have equal numbers of input ports and output ports, and a coupler with 2n ports is commonly referred to as a n.times.n coupler. In the case where the hybrid order n is a power of 2, such higher-order couplers can be synthesized from combinations of 2.times.2 hybrid couplers interconnected by transmission lines.
In synthesizing higher-order couplers from 2.times.2 hybrid couplers, the transmission lines interconnecting the 2.times.2 hybrid couplers essentially cross one another. With the simplest higher-order coupler, the hybrid order n is the second power of 2 and only four 2.times.2 hybrid couplers are necessary to provide a 4.times.4 coupler. This arrangement only incurs one "cross-over" between the transmission lines and it is known to rearrange the positions of the four 2.times.2 hybrid couplers to avoid this single "cross-over".
Multi-port couplers of even higher orders can be synthesized from 2.times.2 hybrid couplers to give an n.times.n coupler where n=2.sup.(2+p) and p is a whole number. Thus, when p=1 an 8.times.8 coupler can be achieved, when p=2 a 16.times.16 coupler, when p=3 a 32.times.32 coupler, and so on. Existing 8.times.8 couplers involve many cross-overs with the result that the transmission lines become a complex multi-layer strucrure.
Such cross-overs in the transmission lines may be implemented in various ways. For example, in stripline, microstrip and similar realizations, the 2.times.2 hybrid couplers can be fitted with connectors and external semi-rigid cables can be used for the transmission lines. In microstrip realizations, bridges of wire, foil or cable can be used. In "square-ax" realizations, bridging devices can be used. In waveguide realizations, combinations of waveguide bends can be used. Also multi-layer microstrip or stripline devices could be designed.
In all of the above realizations, the requirement for cross-overs incurs penalties in the mass, size and complexity of any synthesized multi-port coupler in which n=2.sup.(2+p), and such penalties are problematic in satellite applications where lightness, smallness and simplicity are important.
It is an object of the present invention to provide a multi-port microwave coupler where n=2.sup.p .times.3.sup.q with p and q as whole numbers. The simplest hybrid coupler of this definition is a 6.times.6 coupler which is achieved when p=1 and q=1. It is an ancillary object of this invention to minimize the number of cross-overs in such multi-port microwave couplers.